Fiber optical communication receivers determine content of information on an optical link (such as fiber optical interconnect) by a reference signal. This reference signal is also known as a decision level signal. The reference signal allows an input amplifier or a comparator to determine if the data on the optical link is a logical one or a logical zero. The reference signal is generated by a feedback mechanism in an optical receiver which raises or lowers the reference signal level based on the incoming data on the optical link.
FIG. 1A shows a traditional optical receiver 100 with a closed loop feedback system to generate the reference signal. The receiver includes an amplifier 106 that compares incoming data In with the reference signal level and generates amplified outputs Outp and Outn. Output of the amplifier 106 may be further amplified by amplifiers 107 and 108. The data received by the amplifier 106 comes from an optical link 103 and is converted to an electrical signal by an optical-to-electrical converter 104. This data is DC-balanced by an encoder 102 in a transmitter 101. The DC-balanced data is generated by encoding a specific data pattern on the data received by the amplifier 106 so that the data has nearly equal number of logical ones and logical zeros. The encoding is generally performed by the transmitter 101 transmitting the optical data to the optical link 103. This DC-balanced data is filtered by a feedback mechanism having a low pass filter (LPF) 110 followed by an amplifier (or gain stage) 111. The LPF 110 receives the amplified electrical signal from amplifier 108 and creates a low cutoff frequency (e.g., near DC level) in the overall receiver transfer function. The output of the LPF 110 is amplified by amplifier 111 resulting in the reference signal.
With process, voltage, and/or temperature variations in the optical receiver (104-111), the LPF 110 cutoff frequency may drift away from its near DC level and become higher than the near DC cutoff level. For example, the cutoff frequency may become more than the Nyquist frequency. Such a high cutoff frequency of the LPF 110 may cause the DC drift in the reference signal level. FIG. 1B shows a waveform 120 with a drifting reference signal 122 generated by the optical receiver 100 of FIG. 1A. A drifting reference signal means that the reference signal 122 is no longer near the mid-level of the electrical signal 121, but drifts away from the mid-level. The drift may cause the amplifier 106 of FIG. 1A to output incorrect data because the reference signal may be too low or too high from the mid-level of the electrical signal.
Furthermore, since the data on the optical link 103 of FIG. 1A is encoded by the DC-balanced data, the data is decoded after the amplifiers 106-108 by a corresponding DC-balance decoder 109 for further signal processing downstream. The data coding (encoding and decoding with DC-balanced data) is performed by coding schemes such as 8b/10b and 64b/66b coding schemes. As mentioned above, these coding schemes are used in the feedback system (106→107→108→110→111→106) of the optical receiver to generate the near mid-level (of the electrical signal) reference signal and thus require logic which consumes area and power.
Additionally, traditional optical systems use AC-coupling capacitors such as 105 in FIG. 1A between the transmitter 101 and the receiver (104 111). These AC-coupling capacitors occupy large space and add to the cost of the optical system. Moreover, the AC-coupling capacitors contribute to the DC drift of the reference signal.